1. Field of the Invention
The present invention relates to a steel composition useful in the manufacture of steel parts. More especially, the invention concerns an improved composition for fracture splitting forged steel assemblies, such as for use in connecting rods in internal combustion engines.
2. Description of Related Art
Conventional forged steel connecting rods are usually produced as a one piece forging. During subsequent machining processes the "big end" is split by a machining process such as sawing, broaching, etc. This results in a connecting rod and "cap" which after further drilling and machining operations, can be bolted together to enable the connecting rod to be mated with the crankshaft and bearing shells on assembly of the engine.
Splitting the connecting rod by a fracturing process instead of cutting enables the bolt holes to be drilled prior to splitting, enabling fewer operations to be used. Conventional splitting by machining also requires several further machining operations to be carried out to ensure that the connecting rod and cap can subsequently be relocated precisely on assembly. The use of fracture splitting produces surfaces with a unique topography, which can be relocated precisely on refitting without need for machining.
The overall benefit of fracture splitting therefore is to reduce material loss and to eliminate several machining operations during connecting rod manufacture. Reducing the machining operations results in savings of energy, time, labour, tooling investment and floor space.
The predominant connecting rod materials used in internal combustion engines are wrought steel forgings, cast iron and sintered powder forgings. Steel forgings, whether heat treated or directly air cooled after forging, exhibit higher ductility than cast iron or sintered powder forgings. These properties can give rise to disadvantages when forged steel connecting rods are fracture split, compared to cast iron or sintered powder forgings. The higher ductility results in more deformation of the connecting rod and cap during fracture splitting which can result in deformation of the bolt holes, imperfect relocation of the fracture surfaces and a need to remove more material in the final bore machining process.
Despite this, there are strong advantages in using wrought steel due to its improved mechanical properties as compared with cast iron and lower unit cost as compared with sintered powder forging.
The problem inherent in fracture splitting is to obtain a fracture surface which can be mated successfully, without compromising the overall mechanical integrity of the connecting rod. If the material is not brittle, it will be impossible to mate the resultant surfaces. If the material is insufficiently brittle, then some plastic deformation will result leading to a departure from circularity of the bearing assembly.
This problem has been addressed before, and a summary of several known means of overcoming this difficulty is given in EP 167320. This states that embrittlement of material in or around the separating planes may be provided for by material selection, by heat treatment such as hardening of various types, or by cryogenic cooling of the material to reduce its temperature to below the embrittlement point. Other methods of embrittlement proposed in the prior art include the local generation of hydrogen gas by application of acid or electrochemical means, which results in local hydrogen embrittlement of the steel. Finally, U.S. Pat. No. 5,135,587 proposes selection of a pearlitic steel with a grain size grade between 3 and 8 according to ASTM specification E112-8, which is obtain ed by a steel containing 0.60 to 0.75% carbon. This steel is used commercially for fracture split connecting rods in the form of C70 steel, but several users report machinability difficulties. It is usual to provide notches in the forging to act as fracture initiation sites. These can be formed by conventional or laser machining.
Cryogenic methods of achieving temporary embrittlement generally involve dipping the part in liquid nitrogen. This is a very expensive operation and there are practical difficulties in carrying it out in the normal machining environment.
Methods involving acid or electrolytic generation of hydrogen have obvious practical difficulties and dangers and give rise to effluent disposal expenses.
The steels which are commonly employed for fracture split connecting rods have a carbon content around 0.70% and are based upon the composition disclosed in U.S. Pat. No. 5,135,587. Further details can be found in M A Olaniran and C A Stickels: "Separation of Forged Steel Connecting Rods and Caps by Fracture Splitting"; SAE Technical Paper 930033, 1993. In Europe, steels of this type are normally referred to by the "Kurznamen" Code as C70S6. The mechanical properties of connecting rods in this grade are developed by controlled air cooling after forging, eliminating the need for heat treatment. The main disadvantage of the current C70S6 grade is the relatively poor machinability compared to other air cooled steels which normally have a lower carbon content. This is attributable to the higher content of the more abrasive carbides resulting from a fully pearlitic microstructure. This microstructure is necessary to facilitate fracture splitting.
C70S6 usually have a composition generally as follows:
C 0.65 to 0.75 wt % Si 0.15 to 0.40 wt % Mn 0.4 to 0.60 wt % P up to 0.045 wt % S 0.050 to 0.080 wt % Cr up to 0.20 wt % Mo up to 0.06 wt % Ni up to 0.08 wt % Cu up to 0.40 wt % Sn up to 0.04 wt % Al up to 0.010 wt % V 0.030 to 0.060 wt % N up to 0.016 wt %
This is, however, a summary. Individual steels employ specific compositions lying within narrower or wider bands.
This can result in tensile strengths of 850 to 1000 N/mm.sup.2 with 0.2% proof strengths over 550 N/mm.sup.2. Typical elongation and reduction of area values are 8-12% and 20-30% respectively. Impact energies are typically 10 J with a 2 mm V notch. Hardness generally lies in the 220-310 HB range.
One measure of machinability is V.sub.20, the cutting speed at which a 20 minute tool life is achieved on an unlubricated single-point turning test with high speed tools. The higher the V.sub.20 value, the better the machinability. A plot of V.sub.20 results against hardness is given in FIG. 1 for typical microalloyed steels and two examples of C70S6 grade. It can be seen that the C70S6 gives a poor machinability, at the bottom of the microalloy steel "scatter band".